A customer, Phoebe, confided in us that her Golden Retriever (Maggie) began sobbing to herself progressively over two weeks. She found something amiss and consulted a veterinarian. The prognosis confirmed her fears: Maggie had an aggressive form of lymphoma, a cancer of the white blood cells.
Phoebe knew the toll that lymphoma could take. Four years earlier her father suffered from and died of it.
Fur parents share their homes, their exercise habits and sometimes even their food with their four-legged companions. And increasingly, they are sharing the same diseases: dogs and cats suffer from obesity, diabetes, heart disease, cancer, thyroid disorders and asthma, just like humans.
Now researchers are examining the role that pollutants and other environmental factors may play in these dual diseases. Doctors and veterinarians have begun to work together to investigate common risk factors, such as pesticides, air pollutants, cigarette smoke and household chemicals.
Our animal companions, like many young children, often have higher exposures to lawn and garden pesticides and to household chemicals that can accumulate in dust or on carpets. Because our animal companions share our environments, they are exposed to many of the same pollutants as us.
Scientific research is beginning to reveal some links between their environment and their health. Lawn care chemicals may increase the risk of canine lymphoma and bladder cancer. Cats exposed to flame retardants have a higher rate of thyroid disease, according to one study.
Studies in animal companions can never replace studies in humans, but they can present corroborating evidence. Linking pollutants to human health effects can prove controversial, but if we can find the same links in dogs or cats, that can have a powerful effect as it is one more piece of evidence that the link is a real one.
Phoebe will never know what caused Maggie’s lymphoma. Golden retrievers generally have a high rate of cancer, most likely for genetic reasons. But some research suggests that environmental chemicals may play a role in the development of lymphoma in dogs.
Researchers at the University of Massachusetts and the Tufts University Cummings School of Veterinary Medicine questioned the owners of more than 700 dogs about use of pesticides. Roughly one-third of the dogs had been diagnosed with canine malignant lymphoma, while the other two-thirds had either benign tumors or were undergoing non-cancer surgeries.
Animal companions whose parents reported use of professionally applied lawn pesticides and harsh floor detergents were 70 per cent more likely to have lymphoma, according to the study published in the journal Environmental Research in January.
Dogs also were at higher risk of lymphoma if their human parents used self-applied insect growth regulators on their yards. In addition, dogs exposed to flea powders, sprays and on-spot treatments were more likely to develop lymphoma than those whose parents did not use them. Further on this, Scottish Terriers exposed to certain herbicides, were more than four times likely to develop bladder cancer than those whose yards were untreated, according to a 2004 study by Purdue University veterinarians.
Results of other studies have been mixed, with some showing an increased lymphoma risk in animal companions exposed to lawn chemicals and others finding no link. Malignant lymphoma in dogs closely resembles non-Hodgkin’s lymphoma. More than 4,000 Australians a year are diagnosed with the disease, making it the sixth most common cause of cancer death in Australia.
The close interaction and shared household environments of animal companions and their human owners provides a unique opportunity for evaluating how herbicide and pesticide exposure may contribute to human non-Hodgkin’s lymphoma.
Pesticides may increase the risk of the disease in people, too. Last year, Danish researchers found that people with high levels of DDT and other organochlorine pesticides were more likely to develop non-Hodgkin’s lymphoma years later. Clearly dogs are not humans, but physiologically speaking, they are very similar.
The most heartbreaking thing is their short lifespan. It is also what makes them useful models for human disease. Because dogs live accelerated lives compared with humans, researchers can gather information on a lifetime of exposure much more quickly than in people.
Using animals as sentinels for human health is not a new concept. More than 100 years ago, miners took caged canaries into coal mines to warn them of toxic gases.
In the 1950s, thousands of people in Japan died or suffered serious effects from eating mercury-poisoned fish from Minamata Bay. Locals had first noticed strange neurological symptoms in cats, which they described as dancing in the streets before collapsing and dying.
Animal companions also played an important role in drawing a link between asbestos and mesothelioma. In the 1980s, researchers found high levels of asbestos fibres in the lungs of pet dogs diagnosed with the lung disease. The finding helped increase understanding of the threats that asbestos posed to people.
More recently, researchers have found that ozone, the main ingredient of ground-level smog, may contribute to asthma in cats, and household tobacco smoke may be a risk factor for nose, throat and lung cancers in dogs.
A rise in hyperthyroidism in cats also has been linked to brominated flame retardants, which are used in upholstery and electronics and contaminate dust and canned cat foods. Cats with overactive thyroids – which can lead to weight loss, increased appetite, hyperactivity and death – had higher blood levels of the chemicals.
Because of their meticulous grooming habits, cats may ingest a lot of dust. The link to hyperthyroidism in felines should be alarming to parents of crawling toddlers who explore their environments by putting everything in their mouths.
Toddlers with high exposure to the flame retardants have lower IQs, according to one study. The chemicals also have been linked to altered thyroid hormones in pregnant women, which might harm a baby’s brain development.
Looking at the way environmental pollutants might interact with genetics in animal breeds susceptible to certain diseases may benefit human health as well. We know something about their breed history and susceptibility to certain diseases, which may make it easier to tease out gene-environment interactions.
The functions of certain genes are very similar in dogs and humans so what can be learnt from pets may also be applicable to humans. Dogs and cats develop diseases spontaneously for many of the same reasons people do, which means experts can predict from pets how a new drug may act in humans.
Mouse models are really important in the development of new treatments, but a step has been skipped when we take a drug from lab animals to humans without first looking to veterinary patients.
In Maggie’s case, Phoebe and her vet looked first to human studies to form a treatment plan. Maggie received chemotherapy and experimental high-dose vitamin C injections, a treatment that Phoebe had uncovered while researching options for her father. If it worked in humans, it might work for dogs…
Less than two months after being diagnosed with canine lymphoma, Maggie died.
Phoebe was not aware of the link between floor detergents and lymphoma in dogs, but, she has always been really careful about chemicals…